It is known that a diffusion constant and a hydrodynamic radius (particle size) of a fine particle having a particle size of approximately 1 nanometer (nm) to 3 micrometers (μm) can be quantified by a dynamic light scattering (DLS) method [Non-patent Literature 1]. In a case where laser light that is highly coherent is emitted to a solution sample obtained by dispersing particles into a solution, scattering of the laser light is observed depending on a size and a shape of the particles in the solution, and an intensity of the laser light changes over time in accordance with motion of the particles. In this case, it is empirically and theoretically clear that Brownian motion of a smaller particle can be seen as the intensity more rapidly declines over time in a time-intensity correlation function recorded in a photon correlator (correlator), whereas Brownian motion of a larger particle can be seen as the intensity more slowly declines over time in the time-intensity correlation function. As a result, motion and a particle size of a particle are analyzed through analysis of a relaxation time of a correlation function. Unlike, for example, an electron microscope, the dynamic light scattering method has an advantage of allowing an “as-is” state in which particles move in a solution (e.g., a state in which the particles swell, or a state in which the particles are weakly bound together) to be observed on a real-time basis. Note, however, that the dynamic light scattering method has the following problem. Specifically, according to the dynamic light scattering method, only a sample that is relatively transparent to laser light is analyzed, and it is difficult to carry out measurement with respect to a turbid sample, which is less light-transmissive.
In order to solve the problem, the inventors of the present invention have developed a dynamic ultrasound scattering (DSS) method in which attention is focused on similarity in wave between light and an ultrasonic wave. In the dynamic ultrasound scattering method, an ultrasonic wave that (i) is different from visible light in which a photon itself propagates through a space and (ii) is a wave in which a vibration is transmitted is used as a wave source. Thus, according to the dynamic ultrasound scattering method, difficulty in light transmission depending on, for example, whether a sample is colored or uncolored does not matter. The dynamic ultrasound scattering method was proposed by a Canadian group of John Page so that motion of a particle having a relatively large particle size of a millimeter to a submillimeter (approximately 1/10 of the millimeter) was studied [Non-patent Literature 2 and Non-patent Literature 3]. Page et al. used an ultrasonic wave of several megahertz (e.g., 2 MHz) to carry out complicated fluid analysis with respect to a fluidized bed and a Couette system. Thereafter, in order to apply the dynamic ultrasound scattering method not to analysis of a large particle as in the analysis carried out by Page et al., but to analysis of a smaller particle, the inventors of the present invention used an ultrasonic wave having a high frequency of 20 MHz to develop a “high frequency” dynamic ultrasound scattering method that is applicable to a fine particle having 3 micrometers to 32 micrometers As a result, validity of experimental results was shown by evaluating a sedimentation velocity of a fine particle and calculating a particle size in accordance with the evaluation [Non-patent Literature 4, Non-patent Literature 5, and Patent Literature 1]. Further, not only an average of velocities of sedimenting fine particles but also a change in velocity in accordance with a location of a particle and a time was quantified, and a relationship between (a) such a fluctuation in velocity and (b) a particle size was also analyzed by utilizing a hydrodynamic method [Non-patent Literature 5, Non-patent Literature 6, and Patent Literature 2].
In addition, a method for calculating a particle size of not only a sedimenting fine particle but also a fine particle in Brownian motion by use of a scattering amplitude of a real number which scattering amplitude is generated by an ultrasonic pulse is disclosed [Non-patent Literature 7].